Aqueous fluororesin dispersions comprising a melt-processable fluororesin can be molded into films excellent in chemical stability, nonstickiness, weathering resistance and other characteristics by the coating, impregnation or other method and therefore are widely used in such fields of application as cooking utensils, pipe linings and impregnated glass cloth membranes. In these applications, it is preferred that the aqueous fluororesin dispersions comprising a melt-processable fluororesin be high in fluororesin concentration; therefore, those products obtained by polymerization of a fluorinated monomer or monomers in an aqueous medium in the presence of a fluorinated surfactant and the subsequent concentration are generally used. However, it is desirable from the cost viewpoint to remove the fluorinated surfactant from the aqueous fluororesin dispersions.
A method so far proposed for removing the fluorinated surfactant from aqueous fluororesin dispersions comprises repeating substantially three times the phase separation procedure in the presence of a nonionic surfactant and an electrolyte, wherein, each time, the supernatant is separated and the lower phase is recovered (cf. e.g. Patent Document 1 and Patent Document 2). However, these documents make no substantial disclosure about a concentration, by phase separation, of aqueous fluororesin dispersions comprising a melt-processable fluororesin.
In Patent Document 3 and Patent Document 4, there are described aqueous fluororesin dispersions reduced in viscosity by addition of a nonfluorinated anionic surfactant following the reduction in PFOA content by means of an anion exchange resin. Patent Document 5 describes a method of reducing the fluorinated surfactant content by bringing an aqueous fluororesin dispersion having a certain level of conductivity into contact with an anion exchange resin. However, these documents do not make any mention of the concentration by phase separation.    Patent Document 1: Japanese Kokai Publication 2005-126715    Patent Document 2: Japanese Kokai Publication 2005-171250    Patent Document 3: United States Patent Application Publication 2004/186219    Patent Document 4: United States Patent Application Publication 2004/171736    Patent Document 5: United States Patent Application Publication 2004/143052